This invention relates to a method for the continuous sterilization or disinfection of liquids or liquid streams by combining the effect of ultraviolet (UV) energy with turbulence and aeration resulting from the introduction of air bubbles in the liquid and to the apparatus for carrying out this method.
Ultraviolet radiation, whose main wave length are of the order of between 1800 Angstroms and 4000 Angstroms, has been used in the past for water purification when it was dangerour or not practical to chlorinate or chemically treat the water. For example, ultraviolet sterilizers are used to purify tanks of water inhabited by fish or other aquatic life. In general, water is removed from the tank, by a pump and passed through an enclosure containing an ultraviolet light source after which the water returns to the tank. After a period of time the circulation of water through the ultraviolet purifier will reduce the number of bacteria within the tank water to minimum value expressed as the number or organisms per milliliter of water. Further reduction of bacterial can be achieved only by increasing the intensity of the ultraviolet radiation or by improving the efficiency of the system.
The UV energy falling on a surface at a given distance from an ultraviolet lamp is expressed in microwatts per square centimeter. The closer the surface is to the lamp the greater the intensity of the ultraviolet energy that falls on this surface.
This UV energy must be applied to a bacteria for a given amount of time in order to destroy it. For example, the bacillus anthracis requires an exposure of 8700 microwatt seconds per square centimeter for complete destruction. This means that bacteria can be destroyed by high energy for a short time or low energy for a long time.
In a closed system where the liquid is returned to the tank after purification equilibrium is reached between the rate at which bacteria are destroyed and the rate of which the bacteria reproduce themselves. If the flow through the ultraviolet purification chamber is reduced to increase the exposure time, the resulting increase in kill rate may be compensated for by an increased breeding time within the tank.
In liquids there is a critical distance from the ultraviolet lamp at which 90% of the germicidal energy has been absorbed. This leaves 10% of the energy to be transmitted to the liquid beyond that distance and to the absorbent walls of the enclosure. The distance for a 90% absorption, called the effective depth of penetration, may vary upwards from a few thousands of an inch in milk and serums to 5 inches for some drinking water. For this reason a certain amount of the liquid that passes through the ultraviolet chamber is shielded from the sterilization effect of the lamp by intervening liquid, thus reducing the efficiency of the purifier.
The typical sterilizer in use today depends upon baffles within the liquid chamber to cause turbulance, and direct as much of the flow as possible to within the effective germicidal range of the UV lamp. Because the baffles are not a very efficient means of controlling the liquid flow, much of the liquid that passes through the sterilizer if untreated by the radiation. This requires recirculation for complete sterilization and the consequence unnecessary movement of liquid is that not being treated.
If the UV lamp is immersed in the liquid, the cooling effect of the liquid can greatly reduce the UV energy output of the lamp.
It is an object of this invention to provide a means of achieving a more efficient sterilization of liquids that pass under ultraviolet germicidal lamps.
It is also an object of this invention to reduce the liquid flow rate required by commercial ultraviolet sterilizers without lowering the percent kill of bacteria.
A further object of this invention to overcome the reduced sterilizer efficiency caused by the cooling effect of liquid in contact with the UV lamp.
A still further object of this invention is to eliminate slime buildup on the outside surface of the UV lamp.
Another object of the invention is to prevent spattering of the UV lamp caused by bursting liquid bubbles.
It is also an object of this invention to reduce the electrical hazard of ultraviolet sterilizers.
These and other objects, advantages, features and uses of the present invention will be apparent during the course of the following discussion.